The invention relates to a tensioner with a torsional spring that biases the position of a pivot-arm to which a belt engaging pulley is attached, but more particularly, the invention relates to a tensioner of the "Zed" type where a radial center plane for the pulley is laterally offset from an axis for a pivot-pin and about which the pivot-arm oscillates.
An example of a "Zed" type tensioner is disclosed in U.S. Pat. No. 4,473,362 where the tensioner has a pivot-arm attached to an offset cylindrical member that supports the pivot-arm and rotates about a pivot secured to a base. A torsional spring is used where one end of the spring is constrained at two points by a connection to the pivot-arm, and the other end of the spring is constrained by a connection at the base. A single sleeve-type bushing on the pivot has a bearing surface that supports the cylindrical member connected to the pivot-arm. A housing surrounds and defines an annulus for the spring. The stacked annular walls of the tensioner's spring housing together with a cylindrical shaped damping mechanism located radially inwardly of the spring, tend to make the housing of the tensioner bulky for a tensioner that imparts modest tension (e.g. 50 lbs.) and damping (e.g. 30-40%) into a power transmission belt of a front-end accessory belt drive. Another problem associated with the '362 tensioner is that unequal pressure loads introduced to a single bearing surface of the bushing can result in uneven bushing wear and attendant pulley misalignment.
A tensioner design that reduces bulk while providing comparable belt tensioning capability (e.g. 50 lbs. belt tension) with a comparable pivot-arm length, is disclosed in U.S. Pat. Nos. 5,449,328 and 5,458,541 and which have the same assignee (INA Walslager Schefseffler Kg). The "INA" tensioners are more compact than the tensioner of the '362 patent in that the INA tensionsers exclude the requirement for stacked annular walls at a spring housing. A representation of the INA tensioners as disclosed in the '328 and '541 patents is herein represented by FIG. 6.
Referring to FIG. 6, the prior art tensioner includes a pivot-arm, a pivot-pin connected on one side of the pivot-arm, a base member of generally "tube-pan" shape with two bushings that support the pivot-pin, a disc type damping mechanism, and a torsional spring with one end constrained at two points by the pivot-arm, and an opposite end constrained at two points by the base. The points for constraining the ends of the torsional spring are not used in conjunction with a mechanism to generate a force to help balance a hub load which must be supported by the bushings. The base member includes a cantilevered inner wall, a cantilevered outer wall and a bottom wall interconnecting the inner and outer walls. An annular spring receiving cavity is defined between the inner and outer walls. The bottom wall of the base faces an open end of the base member located next to the pivot-arm. With such an arrangement, the pivot-pin is supported in a cantilevered fashion by the inner wall of the base and wherein the bottom wall, which supports the cantilevered inner wall, is spaced a maximum distance away from a radial center plane of the pulley. The spacing of the bottom wall impairs rigidity of the inner wall for supporting the pivot-pin and maintaining good pulley alignment. The constrained connection of the spring ends to the pivot-arm and base results in a bearing loading that permits the pivot-pin to "cock" or become misaligned with bearing wear because loads to the bearing surfaces are in opposite directions and are significantly higher than a load imparted at the pulley by a tenionsed belt.
While the INA tensioner is of a good compact design for reduced bulk it also has inherent alignment and stiffness problems due to: the orientation of the base with its open-end facing the pivot-arm and its bottom wall spaced a maximum distance from the pulley; the spring hook-up at one end with two points directly constrained by the pivot-arm and at the other end with two points directly constrained to the base; and a flat disc damping mechanism operative in conjunction with a compressive force generated by the torsion spring that allows axial translation of the pulley with disc wear and thus, pulley misalignment.
In response to belt force BF' at the pulley, the spaced bushings are loaded BL1, BL2 in opposite directions. As the bushings wear, the pivot-pin will cock with attendant pulley misalignment. As the friction surface of the disc type damping mechanism wears, the pulley will move in a direction PM resulting in additional pulley misalignment.